1. Field of the Invention
The present invention relates to a multi-axis magnetic lens and variants thereof which can focus a plurality of charged particle beams simultaneously and individually. More particularly, it relates to an apparatus of charged particle beam, which can perform multi-functions for observing a specimen surface simultaneously or in a programmed series.
2. Description of the Prior Art
For manufacturing semiconductor IC chips, pattern defects and/or uninvited particles (residuals) occur on wafer/mask surfaces during fabrication processes, which reduce the yield to a great degree. As smaller and smaller critical dimensions of patterns on wafer/mask are required, electron beam (e-beam) tools for defect/particle inspection and defect/particle review have been widely used for the yield management due to their higher imaging resolutions than the corresponding optical tools.
The defect/particle inspection and the defect/particle review are much different in requirements for throughput (which determines how many defects can be detected out in unit time) and imaging resolution (which determines the minimum detectable defect size). The e-beam tools fundamentally work on the principle of scanning electron microscope (SEM). Due to the geometric aberrations and electron interactions (Coulomb effect), it is very difficult or even impossible to realize both the inspection and the review in a single e-beam tool. The state-of-art is that defect/particle inspection and defect/particle review are respectively performed by individual tools; i.e. each tool can only perform a specific function. A tool for defect/particle inspection at first detects pattern defects and process-induced particles, and then feeds the inspection results to a tool for defect/particle review. The tool for defect/particle review shows the defects/particles with an imaging resolution higher than that of the tool for defect/particle inspection in order to provide information for further analyzing the root causes of deflects/particles. Consequently, to monitor the quality of a wafer or a mask, the wafer or the mask has to be transferred between the tools for defect/particle inspection and defect/particle review one or more times. The one or more transfers therebetween reduce the monitoring speed and increase the monitoring cost.
To solve the foregoing problem, a promising solution is to incorporate the tools for defect/particle inspection and defect/particle review into one apparatus. In this way, the transfers between different tools will be within a common vacuum space and a limited distance range. Many procedures of moving and protecting wafers/masks for the transfers will be removed. Consequently, compared with the conventional way for the yield management, a single apparatus with multi-functions can provide a much higher throughput with a much lower cost.
Each of the foregoing e-beam tools basically comprises an electron gun unit providing a single electron beam and an imaging system comprising a single-axis magnetic objective lens. If simply putting the tools into one apparatus, the spatial interval between every two adjacent tools must be large enough to physically accommodate two single-axis magnetic objective lenses thereof. Hence, the number of tools available for a wafer or a mask will not be sufficient for mass production, and the transfer distances among the tools will be large. Another way is using a multi-axis magnetic lens to replace the single-axis magnetic objective lenses of all the tools, which can reduce the spatial interval by 50%, thereby almost doubling the number of the available tools.
The idea of using a multi-axis magnetic lens to separately focus a plurality of electron beams parallel to each other was proposed as early as in 1973 by Maekawa et al. in U.S. Pat. No. 3,715,580. FIGS. 1A and 1B respectively illustrate the configuration and the magnetic field distribution of the multi-axis magnetic lens. The multi-axis magnetic lens comprises one common exciting coil 44, one yoke 43, and two parallel magnetic conductor plates 41 and 42 with a plurality of through holes in pairs. When an electric current is exerted into the coil 44, between each pair of coaxial holes, a magnetic axisymmetric field (round lens field) will be generated along the coincident central axes thereof (such as 31 in FIG. 1B), and consequently one magnetic sub-lens such as 30 is formed therebetween. The two magnetic conductor plates 41 and 42 form pole pieces for magnetic sub-lenses 10, 20 and 30 which respectively focus the corresponding charged particle beams 1, 2 and 3.
As shown in FIG. 1B, in terms of Fourier analysis, the magnetic field of each magnetic sub-lens comprises not only the axisymmetric component or called as round-lens field, but also a lot of non-axisymmetric transverse field components or called as high order harmonics, such as dipole field and quadrupole field. Only the round-lens field is necessary for focusing an electron beam, and the other components are undesired due to generating additional aberrations. To compensate the influence of each high order harmonic, at least one additional element generating the same type field is required. However, the additional element obviously increases the volume and the complication of the multi-axis magnetic lens.
Chen et al propose one method in U.S. Pat. Nos. 8,003,953, 8,294,095 and 8,445,862 and the cross reference, which can fundamentally mitigate or even eliminate the high order harmonics per se. As shown in FIG. 2A, the method comprises three steps. The first step is inserting a magnetic ring (such as 12) inside each hole of every magnetic sub-lens (such as 10) with a radial gap (such as 14). The radial gap can be vacuum or filled of non-magnetic or weakly-magnetic material. The radial gap weakens the non-axisymmetric distribution of magnetic scalar potential inside the magnetic ring, and as a result reduces high order harmonics. The second step is extending one of the two magnetic rings of each sub-lens inside the other so as to further reduce the high order harmonics inside the magnetic sub-lens. If the extended magnetic ring fully goes through the hole where the other magnetic ring is inserted, in some cases the other magnetic ring can even be removed for simplification in manufacturing, such as shown in FIG. 2C. The third step is using two magnetic shielding plates respectively above and below the two parallel magnetic conductor plates so as to reduce the high order harmonics outside each magnetic sub-lens.
For each magnetic sub-lens, the magnetic rings inside the upper and lower holes function as pole-pieces and the magnetic field is formed through the magnetic-circuit gap (such as 15 in FIG. 2A) therebetween. The magnetic field distribution along the optical axis (such as 11) of the magnetic sub-lens is determined by the magnetic-circuit gap. Due to using the magnetic rings, the magnetic-circuit gap can be freely formed in any shape and any orientation so as to generate a desired magnetic field distribution for a certain application. In FIG. 2A, each magnetic sub-lens has an axial magnetic-circuit gap and can be used as non-immersion magnetic objective sub-lens or magnetic condenser sub-lens. In FIGS. 2B and 2C, the magnetic sub-lenses have radial magnetic-circuit gaps and can be used as immersion magnetic objective sub-lenses.
The foregoing multi-axis magnetic lenses proposed by Chen et al. are mainly used to an apparatus performing a single function required for observing a specimen surface with high throughput, such as defect/particle inspection or defect/particle review of a wafer or a mask. As mentioned at the beginning, an apparatus with multi-functions required for observing a specimen is needed especially for the yield management in semiconductor manufacturing. Accordingly, this invention will propose such an apparatus which can perform a variety of independent functions required for observing a specimen surface simultaneously or in a programmed series so as to realize an all-round observation of the specimen surface with both a high throughput and a low cost.